Antennas are recognized as being transducers which transduce electromagnetic signals between the guided-wave form, in which the direction of propagation is controlled by a conductive or dielectric waveguide, and a free-space form, in which the propagation takes place in an unguided manner. Those skilled in the art know that many terms associated with antennas are used for historical reasons. For example, at a time at which reception of signals was accomplished only by the use of a long wire connected to the receiver, antenna coupling problems were experienced only when high power was involved, which was the case with transmitting antennas. These antenna coupling problems were discussed in terms of radiation of signals applied to a “feed” point or terminals of the antenna. Only later was it recognized that the radiation pattern and impedance characteristics of antennas were identical regardless of the direction of transduction, but by that time the “feed” terminology was firmly established. Thus, both transmitting and receiving antennas have “feed” points or terminals, and characteristics which are the same. Thus, descriptions of antenna operation may be couched in terms of transmission or reception modes, whichever provides the greater clarity in a given context, with the operation in the other mode being understood from the one description.
Antennas are widely used, to the extent that modern communication and sensing would be unrecognizable without their application. Many antenna types are known, including the long-wire Beverage antenna, the dipole and its monopole-over-ground-plane equivalent. The monopole and dipole antenna linear antennas have well-known radiation and impedance characteristics. Among the radiation pattern characteristics of monopole and dipole antennas are relatively limited bandwidth and relatively low gain, which tend to reduce their usefulness for demanding applications.
The art of arraying of elemental antennas such as monopoles and dipoles has long been used to ameliorate some of the disadvantages of linear antennas. Broadband arrays of dipoles and monopoles are known in the form of one-dimensional or line arrays, which tend to provide greater directivity than a single antenna element. Among the line arrays are log-periodic arrays, in which the dimensions of the constituent antenna elements vary in a monotonic manner along the length of the antenna. Log-periodic arrays, in addition to providing more directive gain than a single linear antenna, also have theoretically unlimited bandwidth. U.S. Pat. No. 5,196,857 issued Mar. 23, 1993 in the name of Chiappetta and U.S. Pat. No. 5,214,439 issued May 25, 1993 in the name of Reed describe log-periodic arrays. Another widely used type of array is the planar array, which is a two-dimensional arraying of elemental antennas. As ordinarily configured, such arrays can provide relatively high directivity in a direction orthogonal to the plane of the array. Those skilled in the art know that many different types of elemental antennas can be arrayed in two dimensions. For example, U.S. Pat. No. 5,258,771 issued Nov. 2, 1993 in the name of Praba describes a two-dimensional array of elemental helix antennas.
For some uses, two-dimensional arrays of antenna elements may include hundreds or even thousands of elemental antennas. The use of elemental antennas in an array may require a transmit-receive “feed” module for each elemental antenna. One arrangement for implementing transmit-receive feed modules for a two-dimensional array of elemental antennas is described in U.S. Pat. No. 5,017,927, issued May 21, 1991 in the name of Agrawal et al. Another feed arrangement for implementation of a two-dimensional antenna array is described in U.S. Pat. No. 5,115,244, issued May 19, 1992 in the name of Freedman et al. Regardless of the type of feed, cost considerations become important when considering arrays of more than a few elements. The cost of the elemental antennas is of more than passing interest when large numbers of elements are to be used. In addition, the complexity of the mounting arrangements for each elemental antenna must be considered. U.S. Pat. No. 5,459,474, issued Oct. 17, 1995 in the name of Mattioli et al. describes an array antenna in which the elemental radiating antennas are horns, and the feed arrangement for each elemental horn includes a movable feed element which is inserted from the rear into the horn.
Simple and or low-cost antennas are desired, especially those which are convenient to array.